It is crucial that the temperature of a work substrate processed by, for instance, a substrate processing apparatus, such as a semiconductor wafer (hereafter may be simply referred to as a “wafer”) be measured with a high degree of accuracy in order to accurately control the shapes, the physical characteristics and the like of films, holes and the like formed on the wafer by executing various types of processing such as film formation and etching. Accordingly, various wafer temperature measurement methods have been proposed in the related art, including the use of a resistance thermometer and the use of a fluorescence thermometer that measures the temperature at the rear surface of the base material.
In recent years, research into temperature measurement methods and temperature measuring apparatuses that enable direct measurement of the wafer temperature, which is difficult with the temperature measurement methods in the related art, has gained significant ground. These technologies pertinent to direct temperature measurement include those disclosed in International Publication No. 03/087744 (Reference Literature 1) and Japanese Laid Open Patent Publication No. 2001-203249 (Reference Literature 2). A specific example of a temperature measuring apparatus enabling such direct measurement of the wafer temperature is now explained in reference to FIGS. 7 and 8. FIG. 7 illustrates the principal of a temperature measuring apparatus in the related art, whereas FIG. 8 is a conceptual diagram of interference waveforms measured with the temperature measuring apparatus.
The temperature measuring apparatus 10 in FIG. 7 is constituted with a low coherence interferometer achieved by adopting the basic principle of, for instance, a Michelson interferometer. The temperature measuring apparatus 10 includes a light source 12 constituted with, for instance, an SLD (super luminescent diode) having low coherence characteristics, a beam splitter 14 that splits the light originating from the light source 12 into two beams, i.e., a reference beam to be radiated onto a reference mirror 20 and a measurement beam to be radiated onto a temperature measurement target 30, the reference mirror 20 drivable along a single direction, with which the optical path length of the reference beam can be varied, and a light receiver 16 that receives the reference beam reflected at the reference mirror 20 and the measurement beam reflected at the temperature measurement target 30 and measures the extent of interference.
In this temperature measuring apparatus 10, the light originating from the light source 12 is split at the beam splitter 14 into two beams, i.e., the reference beam and the measurement beam. The measurement beam is radiated toward the temperature measurement target and is reflected at various layers, whereas the reference beam is radiated toward the reference mirror 20 and is reflected at the mirror surface. Then, both reflected light beams reenter the beam splitter 14, and depending upon the optical path length of the reference beam, the reflected light beams become superimposed upon each other, thereby inducing interference. The resulting interference wave is detected by the light receiver 16.
Accordingly, the reference mirror 20 is driven along the single direction to alter the optical path length of the radiated light for the temperature measurement. Since the coherence length of the light from the light source 12 is small due to the low coherence characteristics of the light source 12, intense interference manifests at a position at which the optical path length of the measurement beam and the optical path length of the reference beam match and the extent of interference is substantially reduced at other positions under normal circumstances. As the reference mirror 20 is driven along, for instance, the forward/backward direction (the direction indicated by the arrows in FIG. 7) and the optical path length of the reference beam is adjusted as described above, the reflected measurement beams from the individual layers (A layer and B layer) at the temperature measurement target with different refractive indices (n1, n2), the reflected reference beam interfere with each other and, as a result, interference waveforms such as those shown in FIG. 8 are detected. Thus, the measurement of the temperature at the temperature measurement target along the depthwise direction is enabled.
As the temperature of the temperature measurement target being heated with a heater or the like changes as shown in FIG. 8, the temperature measurement target expands. At this time, the refractive indices at the various layers at the temperature measurement target 30, too, become altered and, as a result, the interference waveform positions following the temperature change shift relative to the positions prior to the temperature change, which changes the intervals between the individual heat positions. The extent by which the peak positions of the interference waveforms change corresponds to the extent of the temperature change. In addition, the distances between the peak positions of the interference waveform correspond to the distance by which the reference mirror 20 moves. Thus, by accurately measuring the intervals between the peak positions in the interference waveforms based upon the distance by which the reference mirror 20 is displaced, the change in the temperature can be measured.
It is desirable that the temperature be measured at a plurality of measurement points instead of just a single measurement point in order to ensure that the processing within the surface of the wafer including the central area and peripheral areas is controlled with a high degree of consistency. For these purposes, the temperature may be detected at a plurality of measurement points by employing a plurality of temperature measuring apparatuses such as that described above each in correspondence to one of the measurement points.
However, if a plurality of temperature measuring apparatuses the quantity of which matches the number of measurement points are to be operated by moving the reference mirrors in the individual temperature measuring apparatuses to measure the interference waves at the various measurement points, the measurement becomes a laborious process. Furthermore, since matching numbers of light sources 12, reference mirrors 20 and light receivers 16 are required, the cost is bound to increase.
It is to be noted that technologies for enabling temperature measurement at a plurality of measurement points include that disclosed in Japanese Laid Open Patent Publication No. H09-318462 (Reference Literature 3). The publication teaches a system that includes a plurality of sensors each comprising a splitter, a measurement beam sensing fiber, a reference fiber for the reference beam and a reflector, with light originating from a single light source guided into the individual sensors via the respective splitters.
However, the principle of the technology disclosed in Reference Literature 3 described above is fundamentally different from the principle of the apparatus shown in FIG. 7 in that the light from the light source is converted to pulse light via a transmission gate and the pulse light is then allowed to enter the individual sensors. In other words, since the interference is measured by modulating the pulse light and then inputting the modulated pulse light to each sensor, which requires the reflected light to be input via a reception gate to each sensor, the temperatures at various measurement points cannot be measured all at once.